The Concept of Data Types
In programming, data types is an important concept.
To be able to operate on variables, it is important to know something about the type.
Without data types, a computer cannot safely solve this:
Does it make any sense to add "Volvo" to sixteen? Will it produce an error or will it produce a result?
In the second example, since the first operand is a string, all operands are treated as strings.
A string (or a text string) is a series of characters like "John Doe".
Strings are written with quotes. You can use single or double quotes:
You can use quotes inside a string, as long as they don't match the quotes surrounding the string:
You will learn more about strings later in this tutorial.
Numbers can be written with, or without decimals:
Extra large or extra small numbers can be written with scientific (exponential) notation:
You will learn more about numbers later in this tutorial.
Booleans can only have two values: true or false.
Booleans are often used in conditional testing.
You will learn more about conditional testing later in this tutorial.
Array items are separated by commas.
The following code declares (creates) an array called cars, containing three items (car names):
Array indexes are zero-based, which means the first item is , second is , and so on.
You will learn more about arrays later in this tutorial.
Object properties are written as name:value pairs, separated by commas.
The object (person) in the example above has 4 properties: firstName, lastName, age, and eyeColor.
You will learn more about objects later in this tutorial.
The typeof Operator
Any variable can be emptied, by setting the value to undefined. The type will also be undefined.
An empty value has nothing to do with undefined.
An empty string variable has both a value and a type.
You can empty an object by setting it to null:
You can also empty an object by setting it to undefined:
Difference Between Undefined and Null